With the increasing costs associated with animal breeding and artificial insemination, each cow obtained by a dairy producer for production of milk products represents an enormous investment of both time and money. Traditional methods of insuring cow performance have included standard breeding techniques in which sire progenies are studied. Milk production ratings (transmitting abilities) are then used to guide further breeding.
One such particularly successful breeding family is the Holstein line derived from Carlin-M Ivenhoe Bell. It has been estimated currently that more than 25% of the highest total performance index Holstein bulls in the United States are progenies of this individual. This standard technique, however, requires years to evaluate the true genetic value by progeny testing each bull. Many cows must be bred and give birth to offspring. The females must be raised, bred, allowed to give birth and finally milked for a minimum length of time to measure their phenotypic traits.
Further, complex gene action and interactions among genes serve to complicate this objective. Selection based purely on phenotypic characteristics does not efficiently take into account such genetic variability. Thus, it is clear a need exists in the art for a method of genetically evaluating cattle to enable breeders to more accurately select those which not only phenotypically express desirable traits but those which express favorable underlying genetic criteria.
The ability to follow a specific favorable genetic allele involves the identification of a DNA molecular marker for a major effect gene. The marker may be linked to a single gene with a major effect or linked to a number of genes with additive effects. DNA markers have several advantages; segregation is easy to measure and is unambiguous, and DNA markers are co-dominant, i.e., heterozygous and homozygous animals can be distinctively identified. Once a marker system is established selection decisions could be made very easily, since DNA markers can be assayed any time after a blood sample can be collected from the individual infant animal; or even earlier as technology is developing to test embryos in vitro if very early embryos are collected.
For example, currently selected females are superovulated, eggs are collected, in vitro fertilized using semen from superior males and implanted into other females allowing for use of the superior genetics of the female (as well as the male) without having to wait for her to give birth to one calf at a time. This would be a modification of a selection scheme, termed multiple ovulation and embryo transfer (MOET) and can be used with genetic marker technology in that developing blastomeres at the 4-8 cell stage can be assayed by PCR for presence of the marker, and selection decisions made accordingly.
Marker-assisted selection could thus lower the high cost of progeny testing currently used to prove sires, since young bulls that are determined by genetic testing to have undesirable markers would never be progeny tested and young bull progeny could be evaluated immediately after birth for the presence/absence of the marker.
In general the standard methodology includes extraction of DNA, digestion with restriction enzymes, separation of the resulting fragments, hybridization to radio-labeled probe, and finally correlation of identified polymorphisms with milk production traits to identify a marker to aid in selection. A newer technology, using the polymerase chain reaction (PCR*) to amplify the relevant gene fragment for further analysis, has also become available. Such PCR technology speeds the analysis, since (1) less genetic material (blood or serum sample) is necessary; (2) the hybridization step to detect specific gene fragments are avoided; (3) costs of some materials (radioactive label, x-ray film) are eliminated, decreasing overall costs of analysis; and (4) remaining analysis steps are very rapid. Genetic markers have been found which indicate the presence of bovine leukocyte adhesion deficiency, markers have been located within the bovine prolactin gene, and with .kappa. casein which are associated with superior milk products and markers have even been used to detect bovine mitochondrial DNA variants which are associated with improved dairy cow performance.
One attempt to improve resulting cattle performance has included boosting the level of growth hormones via introduction of additional hormones. A notable example is use of bovine growth hormone, which has recently gained FDA approval. This has been made possible by the cloning and isolation of genes that express such proteins and then adding the resulting products of these commercially produced proteins to feeds, drugs, etc. This method of boosting production of essential proteins however is inherently limited by the underlying genetics of the animal and does not offer anything in the way of selection of genetically superior animals for optimum genetic capabilities.
Further public acceptance of injecting growth hormone remains to be seen. Qualified administration of multiple injections keep costs high and animals which are sick cannot be so treated. The results of bovine growth hormone injection include an increase in overall milk production, with no change in milk composition. This is significant because a dairy producer is paid on the basis of three milk characteristics, total volume of milk, total pounds of fat in the milk, and total pounds of protein in the milk, thus quality is as important as quantity. Producers are paid more for protein than fat. Thus it can be seen that there is a continuing need for means of efficiently selecting and breeding cattle for improved milk production without concomitant decrease in milk composition, particularly protein.
It is an object of the present invention to provide a genetic marker within the bovine PIT-1 gene which is indicative of increased milk yield and milk composition.
Yet another object of the present invention is to provide an assay for determining the presence of this genetic marker.
Yet another object is to provide a method of evaluating cattle that increases accuracy of selection and breeding methods.
Yet another object is to provide a PCR amplification test which will greatly expedite the determination of presence of the marker.
Other objectives and advantages of the invention will be apparent from the detailed description of the invention which follows.